CTX_DOC.SNIPPET and CTX_DOC.POLICY_SNIPPET produce a concordance of the document, in which occurrences of the query term are returned with their surrounding text. This result is sometimes known as Key Word in Context, or KWIC, because instead of returning the entire document (with or without the query term highlighted), it returns the query term in text fragments, allowing a user to see it in context. You can control the way the query term is highlighted in the returned fragments.
CTX_DOC.SNIPPET and CTX_DOC.POLICY_SNIPPET are equivalent, except that CTX_DOC.POLICY_SNIPPET does not require an index. CTX_DOC.POLICY_SNIPPET and CTX_DOC.SNIPPET include two new attributes: radius and max_length. The radius attribute specifies the approximate desired length of each segment. Whereas, max_length puts an upper bound on the length of the sum of all segments.
Oracle Text Reference for more information about CTX_DOC.SNIPPET and CTX_DOC.POLICY_SNIPPET